Among the unsolved problems in the transport of nutrients such as sugars and amino acids into vertebrate cells are (1) the role of intracellular compartmentation and (2) the source of energy for the active transport of amino acids. The role of intracellular compartmentation in transported nutrients to their site(s) of utilization will be investigated in muscle, a tissue with both a well-defined compartmental structure and insulin sensitivity. A multifaceted approach to this problem will include studies of steady-state distribution in isotonic and hypertonic solutions, kinetic studies in whole muscle and single muscle fibers to determine both compartment size and the rate of passage of nutrients between compartments, the use of photoaffinity labelling techniques, and localization of metabolically useful nutrient pools through selective loading of defined compartments. The action of insulin in altering functional compartmentalization will be determined. The problem of the source of energy for amino acid transport will be studied in metabolizing ascites cells. Following a study designed to determine whether the C1- distribution ratio is a measure of the membrane potential in this tissue, amino acid transport will be studied under well-defined steady-state ionic conditions. The role of the membrane potential, the K ion gradient, the Na ion gradient and ATP hydrolysis will be investigated in order to determine whether cation gradients provide any or all the energy for amino acid transport in metabolizing cells. In addition we will determine whether nuclear magnetic resonance relaxation mechanisms are altered when nutrient molecules like glycine are accumulated in cells. These studies of the fundamental mechanisms by which nutrient molecules enter cells and reach their sites of metabolic utilization form a necessary basis for understanding the control of nutrient transport in health and disease.